1. Field of the Invention
The invention relates to a vehicle steering system that includes a variable transmission ratio mechanism that is able to change a steered amount of a steered wheel with respect to an operation amount of a steering wheel of a vehicle.
2. Discussion of Background
A conventional vehicle steering system of this type is described in, for example, Japanese Patent Application Publication No. 11-001175 (JP 11-001175 A). In the vehicle steering system described in JP 11-001175 A, a steering shaft is split into a first shaft on the steering wheel side and a second shaft on the steered wheel side. A variable transmission ratio mechanism is provided between the first shaft and the second shaft, and is able to change the transmission ratio between the first shaft and the second shaft. Note that the transmission ratio indicates the ratio of the rotation angle of the second shaft to the rotation angle of the first shaft. With such a vehicle steering system, by changing the transmission ratio on the basis of an operating state of the vehicle, it is possible to change an amount of change in the steered angle of the steered wheels with respect to the operation amount of the steering wheel. Therefore, the drivability of the vehicle improves.
In such a vehicle steering system, for example, if one of the steered wheels runs onto a curb while the vehicle is travelling and then an excess impact load acts on the steered wheel, the impact load is transmitted as reverse input torque to the variable transmission ratio mechanism via a steered system and the second shaft. If the reverse input torque acts on a motor of the variable transmission ratio mechanism and therefore a rotary shaft of the motor rotates relative to the first shaft, the correlation between the steering angle of the steering wheel and the steered angle of the steered wheels may deviate.
Therefore, in JP 11-001175 A, the state of a load of the motor is monitored, and, if the fact that the load of the motor is high is detected, rotation of the rotary shaft of the motor is restricted with the use of a lock mechanism provided in the variable transmission ratio mechanism. In this way, the operation of the variable transmission ratio mechanism is restricted. When reverse input torque acts on the motor and therefore the load of the motor becomes high, the operation of the variable transmission ratio mechanism is restricted. Therefore, the first shaft and the second shaft are directly coupled to each other. Thus, it is possible to suppress a deviation between the steering angle of the steering wheel and the steered angle of the steered wheels.
In the vehicle steering system described in JP 11-001175 A, due to a delay in response of the lock mechanism, there is a slight time lag between when the fact that the load of the motor is high is detected and when the operation of the variable transmission ratio mechanism is actually restricted. If the rotary shaft of the motor rotates relative to the first shaft during the time lag, there may be a deviation between the steering angle of the steering wheel and the steered angle of the steered wheels. This is one of factors that deteriorate the drivability of the vehicle.